This invention concerns an improved method of applying an adhesive to flat carton blanks as well as an apparatus for and a method of converting such and other flat blanks into folding and rigid carton constructions.
Paperboard and synthetic carton blanks are commonly die cut from large sheets or rolls. The sheets or rolls may be printed, embossed, coated, and die cut in a continuous process of which die cutting is the final step. Paperboard blanks are often coated with a varnish during the printing process, whereby the finished surfaces are made more durable and gloss is imparted to them. The varnishes are formulated in varying degrees of hardness and gloss to suit specific applications, and typically comprise hydrocarbon solvents in combination with such solids as nitrocellulose, acrylics, and vinyls, or combinations thereof, in concentrations of 25% to a maximum of 30%.
In such applications as milk cartons, the paperboard is coated with a relatively heavy layer of plastic ranging in thickness between 0.00075 inches (0.002 cm.) and 0.001 inches (0.0025 cm.).
In commercial practice, blanks are converted into carton constructions by machines which fold them into the prescribed form and interlock or bond certain of their overlapping portions. Bonding is accomplished with supplementary ahdesives or by heating to flow the plastic coatings present on the blanks from the time they are manufactured. An example of the latter is milk carton construction wherein ultrasonic energy, applied to the exterior side of one overlapping portion, heats to flow the plastic coating present at the sealing faces of both overlapping portions. Patents representative of this art include Pat. Nos. 3,224,915, 3,468,731, 3,905,280, 3,956,975, 4,251,303, 4,159,220, and 4,264,316. Carton blanks entirely comprised of thermoplastic materials may also be converted to cartons in this manner. Most carton constructions, however, are not intended for uses requiring heavy plastic coatings and, therefore, are commonly bonded where necessary with protein glue, synthetic liquid glue, or normally solid substances heated to flow. In some cases, the adhesive is applied during the manufacture of the blank and reactivated at the time the blank is erected. It is the more general practice, however, to apply the adhesive to the blank immediately prior to the forming process. In all cases of bonding overlapping portions of blanks by adhesive means, it is necessary to hold them together under pressure until sufficient bond has been developed to hold them in place.
Inherent in the processes and procedures described above, as implemented in accordance with the prior art, are costs in time, materials, and energy which may be mitigated by the practice of the present invention.